In the modern world plastics are the material of choice for the manufacture of a seemingly unlimited number of products. These products are produced by a variety of industrial processes, e.g. injection molding, blow molding, extrusion, and 3-D printers. The raw material that is fed into the machines used to produce the final products is a mixture consisting mainly of polymers (called resin in the industry) in the form of small beads and relatively very small amounts (by weight) of colorants and other additives, e.g. UV inhibitors. The colorants and other additives are supplied as masterbatches, which are concentrated mixtures of pigments and/or additives encapsulated during a heat process into a carrier resin which is then cooled and cut into a granular shape.
Herein the term masterbatch is used to refer to both color and additive masterbatches. Herein the terms “bead”, “pellet”, and “granule” are used interchangeably to refer to a small piece of material typically having a weight in the range of 0.01 g-0.04 g.
Many different systems have been described in the prior art for weighing and feeding the pellets of material from a hopper into a process machine. Examples of these systems are described in the following patents to the Applicant of the present invention:
U.S. Pat. No. 6,688,493 teaches a system for feeding portions of material to an injection molding-machine. The system includes a container, a material hopper located inside the container, a load cell coupled to the material hopper, a controller that calculates weight according to the load cell information and commands the motor of the screw feeder to dispense portions of material into the molding-machine.
U.S. Pat. No. 6,966,456 describes a method for using the loss-in-weight method to determine the weight of a portion of material dispensed by the screw in a given time. The method comprises, measuring the initial weight of material in a hopper, dispensing a number of portions—each portion provided by turning the screw at a given speed for a given period of time, measuring the weight of material in the hopper after the number of portions has been dispensed, and dividing the difference between the initial and final weights by the number of portions to the weight of each individual portion. If the weight of the portions varies from a desired value, then the calibration process is repeated by varying the speed of the screw, for example decreasing the rotation speed if the portions are too heavy.
The accuracy of the weight of material fed to a processing machine using a weighing and feed system based on the use of screws or augers decreases dramatically as the weight of material to be added to each batch becomes smaller. When a small weight is needed, e.g. masterbatches, volumetric methods are usually used to achieve the desired weight. Volumetric feeding methods, which are known are however not always reliable because, inter alia, the specific gravity of a masterbatch can change from one supplier to another or between production batches from the same manufacturer. Therefore, frequently a new calibration process, which is time consuming and requires skilled workers, must be carried out for each run to determine the volume of masterbatch that has the required weight.
PCT patent application WO 2015/107517 to the Applicant of the present invention describes a feed system that comprises a feed tube that is capable of feeding very precise amounts of masterbatch to mixers and processing machines.
It is a purpose of the present invention to provide a system that is capable of weighing very light portions of masterbatch and delivering the portions to a processing machine.
Further purposes and advantages of this invention will appear as the description proceeds.